Free Carriers versus Self-Trapped Excitons at Different Facets of Ruddlesden–Popper Two-Dimensional Lead Halide Perovskite Single Crystals

Abstract

The physical origin of sub-band gap photoluminescence in Ruddlesden–Poppers two-dimensional (2D) lead halide perovskites (LHPs) is still under debate. In this paper, we studied the photoluminescence features from two different facets of 2D LHP single crystals: the in-plane facet (IF) containing the 2D inorganic layers and the facet perpendicular to the 2D layers (PF). At the IF, the free carriers (FCs) dominate due to the weak electron–phonon coupling in a symmetric lattice. At the PF, the strain accumulation along the 2D layers enhances the electron–phonon coupling and facilitates self-trapped exciton (STE) formation. The time-resolved PL studies indicate that free carriers (FCs) at the IF can move freely and display the trapping by the intrinsic defects. The STEs at the PF are not likely trapped by the defects due to the reduced mobility. However, with increasing STE density, the STE transport is promoted, enabling the trapping of STE by the intrinsic defects.

Origin and physical effects of edge states in two-dimensional Ruddlesden-Popper perovskites

“…Moreover, the PL emissions of edge states exhibit a significantly slower rise time. This suggests the charge carriers spend a very long time, in nanoseconds, to reach their population maximum than the case of bulk ( Figure 1 C), indicating the carrier transport from the bulk region to the low energy edge states ( Blancon et al., 2017 ; Liang et al., 2021 ; Shi et al., 2019 ; Zhao et al., 2019 ). The lower PL energy and the longer PL lifetime of edge states suggest that edge states provide a direct pathway for dissociating the photogenerated excitons into free carriers even though the average binding energy at a level of a few hundred meV for n ≥ 2.…”

“…For 2D RP perovskites with n = 2, the edge state emission of (BA) 2 MAPb 2 I 7 is absent in some case, but the edge state emission certainly occurs when the A-site cation changes from MA + to FA + ((BA) 2 FAPb 2 I 7 ), which can be attributed to the larger size of FA + cation inducing extra strain at the edge than the case in MA + ( Shi et al., 2019 ). Besides, recent research shows that the peak of edge state emission exhibits less redshift when the ligand is changed from n-butylamine to n-pentylamine for (RNH 3 ) 2 (MA)Pb 2 Br 7 (RNH 3 represents ligand) ( Liang et al., 2021 ). The shift of the PL peak of edge state emission can also be controlled by the thickness of the single crystal; the redshift of edge state emission increases with the increasing thickness of the single crystal ( Shi et al., 2019 ).…”

“…Recently, Liang et al ( Liang et al 2021 ) proposed a new hypothesis about the origin of edge states by studying the PL properties excited from different facets of bulk single crystal (perpendicular facet and in-plane facet, denoted as PF and IF, respectively. Figure 4 A), and they suggested that the self-trapped exciton was the reason of the edge states.…”

Ambient-stable near Unity Photoluminescence Quantum Yield in Two-dimensional Tin Bromide Perovskites

“…The ex independent Eb values imply a favorableelectronic transition of the free exciton state (FE) to the free carrier state (FC) for enabling a favorable electron extraction. 28 The extent of carrier-phonon coupling can be further understood by analyzing the FWHM of the PL emission bands (Figure S9). Although there is no appreciable temperature variation of FWHM of the BE emission bands, the FWHM of STE emission band shows a linear increase with temperature at 360 nm excitation.…”

Transfer Printing of Perovskite Whispering Gallery Mode Laser Cavities by Thermal Release Tape

“…High-quality whispering gallery mode (WGM) perovskite nanocavities have attracted widespread interest for developing low threshold and narrow linewidth lasers due to their unprecedented optoelectronic properties, small sizes and low losses that are important for applications such as light sources for on-chip integration, high-performance lab-free sensing [ 1 – 6 ]. In order to realize high-quality WGM perovskite nanocavities, various methods have been developed including low-temperature solution processable synthesis, chemical vapor deposition methods (CVD) [ 7 , 8 ].…”